Tin oxide (SnO2) is known to be useful for gas sensor applications due to its relatively high gas sensitivity, good stability, and low cost. Crystallite size, microstructure, and surface modification (noble metal loading) play a role in the gas-sensing properties of SnO2. Among these, decreasing the crystalline size of SnO2 is quite effective in improving its gas sensitivity. Tin oxide grains contained in gas sensors typically are <20 nm and stable from thermal growth during the sensing operation at elevated temperatures (300° C.-600° C.). These requirements have been met for sensor devices of the sintered block or thick-film types. Thin film tin oxide sensor fabrication by spin-coating or dip-coating from a colloidal suspension, or sol, of tin oxide has provided films with higher sensitivity and stability.
Doping of tin dioxide with other metal ions results in electronic materials with several desirable properties. One of the most prominent of the doped Tin dioxide materials is antimony-doped tin oxide (ATO). The introduction of antimony (Sb) into the tin oxide lattice is reported to greatly increase the electron conductivity, which renders this material useful as an excellent conductive agent. ATO is transparent throughout the visible region, but reflects/absorbs infrared light. These features make the ATO useful, for example, as transparent electrodes, heat mirrors, and energy storage devices. Surface modified ATO nanoparticles can be combined with a variety of polymeric resins to create optically clear nanocomposites film or laminates which are heat shielding. Nanoparticulate ATO has also been used as electrochromic material for the production of printed displays and anode material in lithium-ion batteries. In addition, ATO has applications in nuclear waste management and is a good catalyst for olefin oxidation.
Tantalum and niobium doped tin dioxide materials display nonlinear electrical properties and are useful as varistor materials.
The preparation of doped tin oxide nanoparticles with different shape, size, conductivity, and degree of agglomeration has been addressed by a large variety of techniques. Top-down milling process of agglomerated nanopowders of doped or undoped tin oxide are energy and time intensive and generating nanoparticles <50 nm is difficult to achieve. Smaller particle size of the nanoparticle can improve the optical clarity and decrease the haze of the final product.
Chemical methods in general can provide nanoparticles with smaller size in form of stable dispersions than the physical methods. The sol-gel, polymeric precursor, and co-precipitation techniques mostly provide either large particles or nanoparticle agglomerates. Calcination steps involved in some of these processes accelerate growth and agglomeration of obtained particle. Much better control over the growth of doped tin oxide particles has been achieved by hydrothermal and solvothermal techniques.
The hydrothermal method does not need a calcination process, and the dispersibility of the particles is greatly improved. The starting materials used in the hydrothermal methods are often soluble metal chloride, nitrate or sulfate salts. In the case of tin SnCl4, SnCl4.5H2O, SnCl2, or SnCl2.2H2O, are commonly used halide precursors where as for antimony SbCl3 and SbCl5 are used. Chlorine has been known to get adsorbed on tin hydroxide and is very difficult to remove, and large amount of product is lost during the repeated washing. The residual chlorine ions also affect the surface and electrical properties. In addition these salts are also corrosive and precautions are needed to avoid contamination or corrosion of stainless steel reactors. New synthetic processes involving benign precursors would be desirable for improving the yield and quality of doped and undoped tin oxide nanoparticles. Dispersions of the nanoparticles instead of agglomerated powders could also decrease post-synthesis processes needed to include them in nanocomposites.
There is a need for a process to prepare doped and undoped tin oxide nanoparticles as a stable dispersion with primary particle sizes <20 nm without the limitation of known processes.